1. Field of Invention
The present invention relates to a slurry composition and method for polishing organic polymer-based ophthalmic substrates.
2. Description of Related Art
A variety of slurry compositions are known in the art for use in polishing organic polymer-based ophthalmic substrates. Such prior art slurry compositions generally consist of abrasive particles dispersed in water and/or other liquids. Commonly utilized abrasive particles include, for example, cerium oxide, aluminum oxide, zirconium oxide, tin oxide, silicon dioxide and titanium oxide. Aluminum oxide slurry is most commonly used for polishing organic polymer-based ophthalmic substrates such as, for example, allyl diglycol carbonate polymer, which is commonly known in the art as CR-39, and other higher index of refraction polycarbonate resins.
Slurry compositions consisting solely of aluminum oxide abrasive particles dispersed in water produce acceptable surface quality when used to polish organic polymer-based ophthalmic substrates, but exhibit unacceptably low polishing efficiency (i.e., a low removal rate). The polishing efficiency of such slurry compositions can be improved by using significantly larger aluminum oxide particles, but this results in unsatisfactory surface quality, i.e. scratches, pit marks, orange peel or similar defects.
Koshiyama et al., U.S. Pat. No. 4,225,349, disclose a polishing composition consisting of calcined alumina (aluminum oxide particles) and an aluminum salt polishing accelerator dispersed in deionized water that accelerates the removal of the CR-39 substrate material without degrading the surface quality. The preferred polishing accelerator identified by Koshiyama et al. is aluminum nitrate.
CR-39, which has an index of refraction (“RI”) of about 1.498, has dominated the market for organic polymer-based ophthalmic substrates for many years. In recent years, however, higher index of refraction organic polymer-based ophthalmic substrates have been developed, and these higher index of refraction substrates are becoming more prevalent in the ophthalmic lens market. The higher index of refraction substrates provide several distinct advantages over CR-39 in ophthalmic lens applications. For example, lenses that are fabricated from RI 1.586 polycarbonate substrates are thinner and lighter in weight than comparable lenses fabricated from CR-39, and further provide improved impact resistance. This makes higher index organic polymer-based ophthalmic substrates more suitable for use in fabricating eyewear worn by children, active adults and in the production of safety glasses. Generally speaking, the higher the index of refraction, the thinner and lighter the ophthalmic lens. Higher index of refraction organic polymer-based ophthalmic substrates allow for the fabrication of eyewear that is functional and fashionable, and which eliminates the “bulbous” or thick “coke-bottle” look often found with lower index of refraction lenses.
Known slurry compositions for use in polishing CR-39 do not provide the desired polishing efficiency, even when a conventional accelerator such as aluminum nitrate is present. A slurry composition that provides high removal efficiency, particularly on higher index of refraction organic polymer-based ophthalmic substrates, is desired.